1. Field of the Invention
The present invention relates to converting mist flow to annular flow in a steam cracking application to enhance the flash drum removal efficiency of non-volatile hydrocarbons.
2. Description of Background and Related Art
Steam cracking has long been used to crack various hydrocarbon feedstocks into olefins. Conventional steam cracking utilizes a furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock is then introduced into the radiant section where the cracking takes place. The resulting olefins leave the furnace for further downstream processing, such as quenching.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking low cost heavy feedstock such as, by way of non-limiting examples, crude oil and atmospheric resid. Crude oil and atmospheric resid contain high molecular weight, non-volatile components with boiling points in excess of 1100° F. (590° C.). The non-volatile, heavy ends of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatiles can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. Additionally, some naphthas are contaminated with crude oil during transport. Conventional pyrolysis furnaces do not have the flexibility to process resids, crudes, or many resid or crude contaminated gas oils or naphthas, which contain a large fraction of heavy non-volatile hydrocarbons.
To solve such coking problem, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 450 to 1100° F. (230 to 600° C.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It provides preheating of heavy feed inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feed with superheated steam and the removal of the residual liquid. The vaporized hydrocarbons are subjected to cracking.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feed in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feed is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for super-heating and cracking.
The present inventors have recognized that in using a flash to separate heavy non-volatile hydrocarbons from the lighter volatile hydrocarbons which can be cracked in the pyrolysis furnace, it is important to maximize the non-volatile hydrocarbon removal efficiency. Otherwise, heavy, coke-forming non-volatile hydrocarbons could be entrained in the vapor phase and carried overhead into the furnace creating coking problems.
It has been found that in the convection section of a steam cracking pyrolysis furnace, a minimum gas flow is required in the piping to achieve good heat transfer and to maintain a film temperature low enough to reduce coking. Typically, a minimum gas flow velocity of about 100 ft/sec (30 m/sec) has been found to be desirable.
When using a flash drum to separate the lighter volatile hydrocarbon as vapor phase from the heavy non-volatile hydrocarbon as liquid phase, the flash stream entering the flash drum usually comprises a vapor phase with liquid (the non-volatile hydrocarbon components) entrained as fine droplets. Therefore, the flash stream is two-phase flow. At the flow velocities required to maintain the required boundary layer film temperature in the piping inside the convection section, this two-phase flow is in a “mist flow” regime. In this mist flow regime, fine droplets comprising non-volatile heavy hydrocarbons are entrained in the vapor phase, which is the volatile hydrocarbons and optionally steam. The two-phase mist flow presents operational problems in the flash drum because at these high gas flow velocities the fine droplets comprising non-volatile hydrocarbons do not coalesce and, therefore, cannot be efficiently removed as liquid phase from the flash drum. It was found that, at a gas flow of 100 feet/second (30 m/s) velocity, the flash drum can only remove heavy non-volatile hydrocarbons at a low efficiency of about 73%.
The present invention provides a process for the effective removal of non-volatile hydrocarbon liquid from the volatile hydrocarbon vapor in the flash drum. The present invention provides a process that converts a “mist flow” regime to an “annular flow” regime and hence significantly enhances the separation of non-volatile and volatile hydrocarbons in the flash drum.
Separate applications, one entitled “PROCESS FOR STEAM CRACKING HEAVY HYDROCARBON FEEDSTOCK”, U.S. application Ser. No. 10/188,461, Family Number 2002B063US, filed Jul. 3, 2002, and one entitled “PROCESS FOR CRACKING HYDROCARBON FEED WITH WATER SUBSTITUTION”, U.S. application Ser. No. 10/188,901, Family Number 2002B091US, filed Jul. 3, 2002, are being concurrently filed herewith and are incorporated herein by reference.